Centrosomes are the major and best-understood microtubule-organizing centers (MTOCs) in animal cells. Mutations in genes that encode key centrosomal proteins impact developmental processes leading to syndromic microcephaly, primordial dwarfisms, and other inherited disorders. A major regulator of microtubules at centrosomes and other MTOCs is ?-tubulin, a highly conserved and tubulin variant that is specialized in catalyzing the initiation of microtubule polymer assembly. ?-tubulin associates with gamma tubulin complex proteins (GCPs) to form a ring complex (?-TuRC). We found two distinct types of ?-TuRCs in Drosophila testis: the conventional one that is essential throughout the organism, and a second uncharacterized one whose unique GCP subunits are expressed in late stage testis and are required for sperm motility. This is the first known case of diversity of ?-TuRC core complexes in one organism. We propose to determine the subunit composition of the new testis-specific ?-TuRC and determine if it can assemble hybrid ?-TuRCs with the conventional GCP proteins. We will ascertain the MT nucleation properties of both ?-TuRCs and determine the role for the novel testis-specific variant in sperm motility. The outcomes of this project will be a new understanding of the functions of this essential MT nucleator complex biochemically and in vivo.